Validate sslContext doesn't override a configured trustManager

[raboof]

Short description

When your GrpcClientSettings specifies both an sslContext and a trustManager, Akka gRPC will use the sslContext to create the TLS connection and ignore the trustManager.

This might be non-obvious, and worst case cause too many CA's to be trusted. Perhaps we should add a validation that checks these settings are not both set. That validation could be in the Settings object (imposing a strict order in which settings are applied) or where the settings are used (causing a somewhat less helpful stack trace).